Study On The Plasticizing, Grafting Modification Of Novel Synthetic Polyisoprene Rubber And Its Structural Evolution In The Kinetic Field

In order to ease the shortage of China’s natural rubber (NR) shortage, vigorously developing synthetic polyisoprene rubber (IR), the alternative products of NR, has become the best choice. However, since IR catalytic systems and techniques are different and its microstructures and performances have sharp differences. To boost the use of IR in rubber industry, it has become the first research direction for this research project to systematically explore the processing characteristics, formula system, and structure-performance relationships of IR. Moreover, with the development of green tire and high-performance conveyor belt, lots of nanoparticles are used in the formula system to provide rubber materials with high strength, high modulus, high elasticity and long service life. The addition of fillers improves the viscosity of rubber compounds significantly, leading to a poor processability; therefore, the processability of rubber filled with lots of fillers should be improved. It is the preferred method to add lots of plasticizers in the formula system to improve the processability of rubber compound. Conventional low-molecular-weight plasticizers and aromatic oil will migrate to the surface of the rubber products, leading to a poor stability of rubber products and serious environmental problem during long-term use. The preparation and application of novel reactive plasticizer, and the effect of plasticizers on the structural evolution of rubber compounds in the kinetic field become the second research direction. In order to make the chemical structure of IR more close to NR, the grafting of oxygen-containing monomer onto IR to increase the physical junction of IR becomes the third research direction.In the first part (Chapter 3,4), we optimized the processing technology and formula system of domestic rare earth polyisoprene rubber (IR70). We also compared the structure and properties of NR, IR70 and Russian titanium polyisoprene rubber (SKI-3). The results showed that IR70 did not need to be masticated in processing. The usage of n-cyclohexyl-2-benzothiazolesulfena--mide alone as the accelerator of IR70 was better. IR70 attained better comprehensive properties after filled with 50 phr of carbon black N234. The comprehensive properties of IR70 and SKI-3 were similar; therefore, IR70 can be used in rubber products in replace of SKI-3.In the second part (Chapter 5), we synthesized liquid polyisoprene rubber (ZLIR) of different molecular weight. The reactivity of ZLIR and kuraray liquid polyisoprene rubber (LIR-50) was characterized by differential scanning calorimetry (DSC) and Soxhlet extraction method. The effects of LIR (including ZLIR and LIR-50) and naphthenic oil (NPO) on the processability of IR70 were characterized by Mooney viscosity tester, capillary rheometer and Haake torque rheometer. The mechanical properties of IR70 plasticized by LIR and NPO were also studied. The results showed that ZLIR acted as a reactive plasticizer to improve the processability of rubber and the CB dispersion during processing. ZLIR participated in the vulcanization reaction and became part of the rubber networks. However, NPO still existed as small molecules in IR vulcanizates, and could be extracted from rubber matrix easily. The mechanical properties and durability of IR plasticized by ZLIR were slightly better than that of IR plasticized by NPO. The effects of self-made ZLIR with number-average molecular weight 50,000 (ZLIR-50) on the mechanical properties of IR were similar to that of LIR-50. ZLIR-50 can be used in replace of LIR-50. ZLIR with number-average molecular weight of 30,000 to 70,000 could all participate in vulcanization. IR70 plasticized by ZLIR-50 attained better comprehensive properties, i.e. better processability and mechanical properties. ZLIR-50 could also be used to replace parts of IR in the IR/CB vulcanizates to improve the processability, broaden the molecular weight distribution index of IR, and achieve a better balance between processability and mechanical properties of IR. Therefore, ZLIR-50 can be used as a novel processing aid and modifier in the rubber industry.In the third part (Chapter 6), the strain-induced crystallization (SIC) characteristics of NR, IR and its samples plasticized by NPO and ZLIR were characterized by in situ synchrotron wide-angle X-ray diffraction (WAXD). The onset strain of crystallization (αc), the relationships of SIC with the upturn of stress, and the effects of plasticizers on the SIC of NR and IR were studied. The results showed that NR had better SIC than IR, i.e. lower αc and higher strain-induced crystallinity. The addition of NPO and ZLIR suppressed the SIC characteristic of NR and IR, i.e. the crystallinity and tensile strength decrease. Compared with NPO, the suppressing effect of ZLIR on the SIC of NR and IR, and the increase of αc for samples plasticized by ZLIR were lower. Samples plasticized by ZLIR had higher strain-induced crystallinity and tensile strength than the samples plasticized by NPO. The strain-induced crystallinity of IR and NR was connected with the upturn of stress during stretching. The stress-strain curves of NR, IR and the plasticized samples upturned until the strain-induced crystallinity was closed to 8%. The molecular mechanism that strain-induced crystals formed quickly, which was ’the slower the stress relaxation, the faster the strain-induced crystals form’, was firstly put forward.In the fourth part (Chapter 7), we explored the suitable monomers and grafting process for the modification of IR. Acrylic acid (AA) was grafted onto the IR by redox initiator, and obtained the grafted product (IR-g-AA). The grafting yield of AA was 2.4%. The thermal stability of IR-g-AA was improved slightly compared with that of IR. The crosslink density of IR increased after grafting modification, and the strain that the stress-curves upturned moved up slightly.